This is a 371 application of PCT/JP99/01598 filed Mar. 29, 1999.
1. Field of the Invention
The present invention relates to a wavelength conversion crystal, and a method and apparatus for generating a laser beam. More particularly, the present invention relates to a novel wavelength conversion crystal useful as a nonlinear optical crystal, and a method and apparatus for generating a laser beam.
2. Description of the Related Art
With the drastic revolution of laser technologies in recent years, it has become a major challenge to perform the wavelength conversion of a near-infrared solid-state laser beam using a nonlinear optical crystal.
A solid-state laser has a narrow spectral bandwidth and stable output, and is easily maintained and feasible for miniaturization, so that it is attracting attention as a means for laser processing and laser-based medical treatments, and also for applications such as surface reforming and optical information processing. In order to capitalize on these beneficial properties of such solid-state lasers, the wavelength conversion technologies have become increasingly important.
An ideal nonlinear optical crystal for such wavelength conversion is required to have; i) a large nonlinear optical constant; ii) a short absorption edge; and iii) an adequate double refraction index. Also, as a crystal, it is further desired to have; iv) superior mechanical properties; from a practical point of view.
The term xe2x80x9ciii) an adequate double refraction indexxe2x80x9d is a double refraction index that satisfies its non-critical phase matching condition under which the wavelength conversion is performed most efficiently. When the double refraction index is smaller than the ideal value, the wavelength conversion would become impossible, and when it is larger, the conversion efficiency would degrade since such a large value results in the departure from the non-critical phase matching condition.
Nonlinear optical crystals have been studied from various viewpoints, and among them, calcium oxyborate-type (COB) crystals are attracting attention.
For example, the non-linearity had been found in GdCa4O(BO3)3:GdCOB by Aka et al., and the growth and optical properties of its single crystal had been reported in 1996. It had been found that this GdCOB;
can be grown by the Cz method, and is non-water-soluble;
has a Vickers hardness of approximately 600 (as hard as quartz);
has doff (at 1064 nm) of 1.3 pm/V (about 3.4 times of KDP);
has a phase matching threshold wavelength of 840 nm; and
is incapable of generating third harmonics of Nd:YAG.
However, the major drawback of this GdCOB is in the fact that its double refraction index is as small as 0.033.
That is, although this GdCa4O(BO3)3(GdCOB) crystal is easy to grow and superior in its mechanical properties, the wavelength it can possibly generate through wavelength conversion is long because its double refraction index is small. Accordingly, the inventors of the present invention have discussed a means to increase this double refraction index, and found that when Gd in a GdCa4O(BO3)3(GdCOB) crystal is replaced by Y, its double refraction index increases. As a result, while a GdCOB crystal can only generate second harmonics of an Nd:YAG laser, the YCOB in which Gd is substituted by Y can generate third harmonics of an Nd:YAG laser.
The inventors of the present invention have already proposed this newly discovered YCOB crystal in a concretive manner.
However, since the arbitrary control over the double refraction indices of COB crystals had been believed to be impossible, the inventors of the present invention extended their discussion over COB as a nonlinear optical crystal for wavelength conversion, and set their objectives to provide a novel technical approach which allows the optimization control over the double refraction indices, as a critical requirement for especially wavelength conversion.
In addition, for wavelength conversion crystals, the conversion to second harmonics has also presented a critical problem.
This problem stems out from the fact that, LBO(LiB3O5) crystal currently used as a wavelength conversion crystal for generating second harmonics of Nd:YAG laser has quality problems and its growing cost is high since this LBO crystal is a water-soluble crystal so that it does not provide a sufficient life span and reliability, and in addition, since it has to be used at a temperature of as high as 148xc2x0 C., and the crystal growth is difficult. Therefore, there has been a demand for the development of a wavelength conversion crystal, which can substitute LBO crystal for generation of second harmonics of Nd:YAG laser. Especially, there has been a strong demand for a wavelength conversion crystal which can generate second harmonics of Nd:YAG laser in a manner in which the non-critical phase matching condition can be satisfied even under a room temperature, which has not yet been implemented in the past.
Furthermore, various types of optical elements for wavelength conversion have previously been proposed. For example, a wavelength conversion element has been used in an ultraviolet laser beam oscillator for converting an infrared beam into an ultraviolet beam. However, in the prior art, a large number of optical elements were required, resulting in complexity in the optical system, and thus, making it difficult to construct a small laser beam oscillator.
Moreover, there had been proposed a crystal element, which allows the generation of second harmonics as well as the oscillation of an infrared laser beam with this one same element. However, it has been difficult to obtain a crystal element capable of generating up to third harmonics.
Accordingly, a novel multifunctional laser beam generator has been desired, which, as a single optical element, has multi-functionality, and is capable of generating second and third harmonics, and is also capable of being implemented in a small ultraviolet laser beam generator.
The present invention first provides a wavelength conversion crystal, which is expressed in the following formula (I); M1xM21xe2x88x92xCa4O(BO3)3; where each of M1 and M2 represents one or more type of different rare earth elements, and 0 less than X less than 1.
More particularly, the present invention provides also a wavelength conversion crystal in which M1 and M2 of the above formula are selected from a group comprising Gd, Y, La and Lu.
As stated above, the present invention is based on an X-ray diffraction observation made on a sintered body, which verified that, for GdCOB expressed by GdCa4O(BO3)3, it is not only possible to replace the Gd with Y, it is also possible to introduce the rare earth elements such as Lu and La etc. into the Gd site so as to change the lattice constant. Since there is a correlation between a lattice constant and a refractive index, the change in the lattice constant would mean a change in the double refraction index of the crystal. Accordingly, the discussion was further extended, and it was found that the double refraction index could be arbitrary controlled by changing the ratios of Gd, Y, Lu and La, and this discovery provided integrity to the present invention. That is, the double refraction index changes in the order of, for example, Lu greater than Y greater than Gd greater than La. As a result, an optimal double refraction index can be obtained for any arbitrary wavelength from third harmonics (355 nm) to second harmonics (532 nm) of Nd:YAG laser, so that the non-critical phase matching condition can always be satisfied.
The present invention secondly provides a novel means for generating second harmonics, which allows an optimization control over the double refraction index of a COB crystal to replace the LBO crystal that has previously been used for second harmonics generation.
That is, the present invention provides a nonlinear optical crystal for second harmonics generation, which is represented by the following formula (II); GdxY1xe2x88x92xCa4O(BO3)3; where 0.01 xe2x89xa6Xxe2x89xa60.35.
The present invention also provides a method for generating laser beam wherein the laser beam is sent through a nonlinear optical crystal represented by this formula (II) to convert it into second harmonics, and an apparatus for generating laser beam comprising this crystal as a means for generating second harmonics.
Furthermore, the present invention also provides a laser beam generator having a nonlinear optical crystal as a means for generating third harmonics.
The present invention thirdly provides, as a novel laser beam generator capable of generating the fundamental wave of laser, and wavelength-converted beams of second and third harmonics, a laser beam generator comprising a nonlinear optical crystal of calcium oxyborate-type crystal containing Gd and Y with Yb or Nd doped thereto, and wherein this crystal element is configured to be able to perform the laser oscillation of the fundamental wave and the generation of second and third harmonics laser beams.